(a) Field of the Invention
The present invention relates to an organic light emitting element, a display device including the same and a method for manufacturing the organic light emitting element.
(b) Description of Related Art
As compared with conventionally used cathode ray tube (CRT) display elements, flat display elements are more power-saving and slimmed down. As a wide variety of information processing apparatuses have been developed in recent years, there is a growing demand for the flat display elements. Examples of the flat display elements include, for example, liquid crystal display elements and electroluminescent display elements (hereinafter referred to as EL display elements). Among them, organic EL display elements have been actively studied because of their features of low voltage drive system, being made of solid material only, quick response and a self-light-emitting property.
The organic EL display elements are classified into those of passive matrix (PM) type and those of active matrix (AM) type depending on the drive system.
A PM organic EL element is driven line by line. Therefore, to realize a PM organic EL element of high luminance, a high instantaneous power must be applied to pixels (light emitting layers). This brings about significant deterioration of the light emitting layers, thereby reducing life of the PM organic EL element. Especially, in an organic EL element including a large number of scanning electrodes (i.e., high definition element), a higher voltage is applied to the pixels. Therefore, the high definition organic EL element is particularly short-life.
On the other hand, since an AM organic EL element includes switching elements (e.g., TFTs) for the respective pixels, switching among the pixels is allowed. For this reason, in principle, there are no limitations on the number of scanning electrodes in the AM organic EL element. Further, every pixel can be driven at almost 100% performance for a single frame.
That is, even if the instantaneous luminance is lower than that of the PM organic EL element, the AM organic EL element allows image display with high luminance and excellent quality. Further, since the AM organic EL element does not require as high instantaneous luminance as the PM organic EL element does, the AM organic EL element offers reduction in drive voltage and long life. Under these circumstances, the AM organic EL element has been actively studied in recent years.
FIG. 14 is a schematic sectional view of a conventional organic EL element 600.
The organic EL element 600 includes a substrate 601 and an organic light emitting layer 603 which is formed above the substrate 601 and sandwiched between a first electrode 602 and a second electrode 604.
The first electrode 602 implants holes into the organic light emitting layer 603, while the second electrode 604 implants electrons into the organic light emitting layer 603. The holes implanted from the first electrode 602 and the electrons implanted from the second electrode 604 are recombined in the organic light emitting layer 603, whereby light emission occurs in the organic light emitting layer 603. The substrate 601 and the first electrode 602 are transparent to light, while the second electrode 604 reflects light. Light emitted by the organic light emitting layer 603 passes through the first electrode 602 and the substrate 601 and is output from the organic EL element 600 (bottom-emission type).
Since the organic EL element 600 is operated by the AM drive system, TFTs and electrodes (pixel electrodes, gate electrodes and source electrodes) are formed on the substrate 601. In general, the TFTs and the electrodes are made of material which is less transparent to light. More specifically, the TFTs are made of silicon which is less transparent to light. Therefore, the AM organic EL element 600 has a problem in that the ratio of a light emitting area to a pixel area (aperture ratio) is low.
The AM organic EL elements are classified into those of current drive system and those of voltage drive system. The current-driven organic EL element reduces variations in display performance of the pixels and effectively prevents deterioration of light emitting material. However, the current-driven organic EL element requires more TFTs to be provided in each pixel than those required in the voltage-driven organic EL element. For this reason, the current-driven organic EL element has a problem of a further decrease of the aperture ratio.
In view of the above problems, top emission organic EL elements have been proposed (for example, see Japanese Unexamined Patent Publication No. 2004-127551). In the top emission organic EL element, the second electrode is transparent to light and the first electrode reflects light. Therefore, light generated in the organic light emitting layer is output from the second electrode side opposite to the substrate side where the TFTs and the electrodes of low light transmittance are provided. Therefore, the top emission organic EL element has a higher aperture ratio than that of the bottom emission organic EL element. That is, the organic EL element is given with high luminance by employing the top emission system.